The present invention relates to a DC-DC converter, for use in various types of electronic equipment, which receives a DC voltage from a battery or the like and supplies a controlled DC voltage to a load. The present invention particularly relates to a DC-DC converter which is capable of quickly responding an abrupt drop in output power (output voltage and/or output current).
Among DC-DC converters which receive a DC voltage from a battery or the like as an input DC source and supply a controlled step-down DC voltage to a load include some DC-DC converters which are structured such that an operation mode is switched in accordance with the state of the load (a light load state or a heavy load state). An operation mode in a light load state herein referred to is an operation mode that electronic equipment is in a standby operation state for instance, and an operation mode in a heavy load state herein referred to is an operation mode that electronic equipment is in a normal operation state for instance. The reason for switching the operation mode in accordance with the state of a load is to reduce the consumption power of the DC-DC converter when the load is light as during a standby. A DC-DC converter having such a structure is described in Japanese Patent Application Laid-Open Gazette No. H11-146637.
FIG. 17 is a circuitry diagram which shows a structure of the conventional DC-DC converter which is described in Japanese Patent Application Laid-Open Gazette No. H11-146637. As shown in FIG. 17, the DC-DC converter, which is connected to an input DC source 301 outputting a DC voltage Vi, comprises an input-side smoothing capacitor 302, a synchronous rectifier circuit 310 and an output-side smoothing capacitor 307. A load 308 is connected to an output terminal of the DC-DC converter.
The synchronous rectifier circuit 310 of the DC-DC converter comprises a main switch 303, a synchronous switch 304, a commutating diode 305, an inductor 306, and a control part 309 which controls turning on and off of the main switch 303 and the synchronous switch 304. By means that the control part 309 switches the main switch 303 and the synchronous switch 304 in synchronization, the DC-DC converter outputs a predetermined DC voltage to the output terminal which is connected to the load 308. The DC-DC converter is structured so that the DC-DC converter switches to the operation mode in the light load state (the standby operation state) or the operation mode in the heavy load state (the normal operation state) in accordance with the state (the light load state or the heavy load state) of the load 308 which is connected to the output terminal.
In such a conventional DC-DC converter shown in FIG. 17, the DC voltage Vi of the input DC source 301 is applied to the synchronous rectifier circuit 310 via the input-side smoothing capacitor 302, and a voltage Vo from the output-side smoothing capacitor 307 is fed as an output DC voltage to the load 308. The control part 309 controls such that the synchronous switch 304 turns off when the main switch 303 is ON but turns on when the main switch 303 is OFF.
When the main switch 303 is ON, the DC voltage Vi of the input DC source 301 is applied to the inductor 306. At this time, a current flows from the input DC source 301 toward the load through the inductor 306 and magnetic energy accumulates in the inductor 306. Next, since the main switch 303 turns off, the synchronous switch 304 turns on and becomes into conduction. As a result, a current flows from the inductor 306 toward the output-side smoothing capacitor 307 through the synchronous switch 304, and the accumulated magnetic energy is released.
As described above, as the magnetic energy is accumulated and released repeatedly in the synchronous rectifier circuit 310, electric power is supplied from the output-side smoothing capacitor 307 to the load 308.
With a duty ratio, that is the on-to-off time of the main switch 303 and the synchronous switch 304, controlled by the control part 309 of the conventional DC-DC converter shown in FIG. 17, the output DC voltage Vo is capable of setting within the range from zero to the input voltage Vi.
A description will now be given on an operation of controlling the duty ratio of the main switch 303 and the synchronous switch 304 in the conventional DC-DC converter having such a structure as above.
FIG. 18 is a voltage waveform diagram which represents respective portions within the conventional DC-DC converter. In FIG. 18, denoted at Vt is a (voltage waveform which is a reference triangular waveform which linearly rises and abruptly drops, and is formed by an oscillating circuit in the control part 309. Denoted at Ve is an error voltage outputted from an error amplifier which is disposed in the control part 309, which is a difference between the output voltage Vo and a reference voltage Vref. Further, in FIG. 18, a first drive signal Vd1 is a signal which is for driving turning on and off of the main switch 303, and a second drive signal Vd2 is a signal which is for driving turning on and off of the synchronous switch 304. The output DC voltage as a target for controlling reaches a desired DC voltage by means that the main switch 303 and the synchronous switch 304 turn on and off in response to the first drive signal Vd1 and the second drive signal Vd2. The first drive signal Vd1 and the second drive signal Vd2 are generated by comparing the reference triangular waveform voltage Vt with the error voltage Ve in the error amplifier of the control part 309.
The error voltage Ve shown in FIG. 18 decreases when the output DC voltage Vo tries to increase as the load 308 becomes light, but increases when the output DC voltage Vo tries to decrease as the load 308 becomes heavy.
Further disposed to the control part 309 is a backward current prevention circuit which detects the value of a current which flows in the synchronous switch 304 when the synchronous switch 304 is ON and accordingly detects a light load state. When a current which flows in the synchronous switch 304 exceeds a predetermined value, the backward current prevention circuit determines that a light load state has occurred and turns off the synchronous switch 304.
As described above, the conventional DC-DC converter is structured such that it is possible to appropriately change the output DC voltage in accordance with the state of the load. In the DC-DC converter, for the purpose of changing the output DC voltage for a DC-DC converter which serves as a DC voltage source, when a reference voltage of this DC-DC converter is changed owing to a signal from the load or in other appropriate conditions, it is desirable that the output DC voltage rapidly responds to a change in reference voltage and becomes a desirable DC voltage.
In the conventional DC-DC converter having such a structure as above, the response speed of the conventional DC-DC converter is dependent upon a changing speed of the error voltage Ve which is outputted from the error amplifier. On the other hand, for the purpose of ensuring safety of a control system in the DC-DC converter, a cut-off frequency of the error amplifier is generally about tenths of a switching frequency which is set to tens to hundreds of kHz, because of a phase compensating capacitor or the like. Hence, a response time of the conventional DC-DC converter is hundreds microseconds in response to a stepwise change in reference voltage, and therefore, it is difficult to ensure a satisfactory response speed to a requirement from the load. In a DC-DC converter having a standby operation mode, even when the load requires to decrease an output DC voltage by changing a reference voltage, the DC-DC converter remains operating in the standby operation mode in a light load state. Such a DC-DC converter therefore has a problem that a period of time for decreasing the output DC voltage is dependent upon a discharge time of discharging from the output-side smoothing capacitor to the load and the response time further slows down.
An object of the present invention is to provide a highly versatile DC-DC converter achieving an excellent response speed by means that power is regenerated on the input side and an energy efficiency accordingly improves in a transient state or at the time of a start-up that output power sharply drops, that is a dive state in response to a load's requirement of reduction of an output DC voltage, etc.